The cheapest petrol will be $139.3¢ at the Caltex Woolworths at South Lake and Osborne Park and the United station in Ellenbrook.

Analysts are warning that the oil price could reach $US100 a barrel on the back of America’s continuing sanctions against Iran, up from about $US73 a barrel.

Members of OPEC have lifted supply but not enough to offset the reduction caused by the absence of Iran from most markets.

US President Donald Trump has criticised OPEC members and spoke with King Salman bin Abdulaziz of Saudi Arabia at the weekend to urge his nation to increase production.

Key speeches from Saudi Arabia and Russia are due this week which may signal whether OPEC will find new ways to push up production.

UBS Group wealth management executive director for commodities, Wayne Gordon, said he believed high prices would persist for at least the rest of the year.

“We’re in a supply-constrained market now, for at least the next quarter into 2019,” he said. “Any geopolitical tension that we don’t foresee coming, with such low spare capacity, is the thing that would push oil to $100 a barrel.”

But on Melbourne radio, Australian Competition and Consumer Commission chairman Rod Sims said motorists were paying too much for petrol.

“In our view, margins are two to three cents a litre too high,” he said. “That’s $400 million to $600 million — that’s a lot of money for Australian motorists.”

Mr Sims said it’s not against the law for consumers to be overcharged for goods, but motorists can work out the best time to buy based on price fluctuations.

“When prices are at bottom of the cycle you’re probably getting petrol below cost and when they’re at the top of the cycle you’re getting ripped off big time,” he said.

“It just fluctuates in a way that irritates the hell out of motorists, but it’s not against the law the way that’s happening.”

Treasurer Josh Frydenberg has said the Federal Government would consider any suggestions to push down prices.

“If they make any recommendations to government then, of course, we will favourably consider those,” he told Nine Network.

“What we understand about our petrol market is that it is driven to some extent by what happens overseas, with the Australian dollar, but also with global oil prices.

“But if companies here are ripping off consumers then we will take action.”

Mr Sims says 35 per cent of the cost of petrol is tax and Australians are paying a fuel excise of 41¢ on top of the GST.

The tax is meant to fund roads but he says it’s questionable whether the taxation level is appropriate.

“The link is so poor, you have no idea whether this level of taxation is the right one to fund the roads,” he said.

“If we could just try and get some better road user charging, so motorists know as they as they pay money, it’s actually going to build roads and they’re not paying more than they should, that’d be a good step. That’s 85 per cent of it.”